The spine includes a series of joints routinely called motion segment units, which is the smallest component of the spine that exhibits kinematic behavior characteristic of the entire spine. The motion segment unit is capable of flexion, extension, lateral bending and translation. The components of each motion segment unit include two adjacent vertebrae and their apophyseal joints, the intervertebral disc, and the connecting ligamentous tissue. Each component of the motion segment unit contributes to the mechanical stability of the joint.
Components of a motion segment that move out of position or become damaged can lead to serious pain and may lead to further injury to other components of the spine. Depending upon the severity of the structural changes that occur, treatment may include fusion, discectomy, laminectomy.
Underlying causes of structural changes in the motion segment unit leading to instability include trauma, degeneration, aging, disease, surgery, and the like. Thus, rigid stabilization of the motion segment unit may be the most important element of a surgical procedure in certain cases (i.e., injuries, deformities, tumors, etc.), whereas it is a complementary element in others (i.e., fusion performed due to degeneration). The purpose of rigid stabilization is the immobilization of a motion segment unit.
As mentioned above, the current surgical techniques typically involve fusing of the unstable motion segment unit and possibly, the removal of ligaments, bone, disc, or combinations thereof included in the unstable motion segment unit prior to fusing. There are several disadvantages to fusion, however. For example, the fusing process results in a permanent or rigid internal fixation of all or part of the intervertebral joints and usually involves metallic rods, plates, and the like for stabilization. In all cases, the systems are intended to rigidly immobilize the motion segment unit to promote fusion within that motion segment unit.
In addition to a loss of mobility, fusion also causes the mobility of the motion segment to be transferred to other motion segments of the spine. The added stresses transferred to motion segments neighboring the fused segment can cause or accelerate degeneration of those segments. One other disadvantage to fusion is that it is an irreversible procedure. In addition, fusion of a motion segments has a clinical success of approximately 70 percent, and often does not alleviate pain experienced by the patient.
Thus, while such fusion systems have been used since the early 1960's, the inherent rigid design have often caused stress concentrations and have directly and indirectly contributed to the degeneration of the joints above and below the fusion site (as well as at the fusion site itself). In addition, rigid, linear bar-like elements eliminate the function of the motion segment unit. Finally, removal of portions of the motion segment unit reduces the amount of support available for the affected motion segment unit.
Fusion procedures can be improved by modifying the load sharing characteristics of the treated spine. Thus, it would be desirable to allow more of a physiologic loading between pedicular fixation and anterior column support. It would also be desirable to have a device that precludes or at least delays the need for fusion for all but the most advanced degeneration of a motion segment, particularly if such a device would allow close to normal motion and pain relief.
Thus, a need exists in the art for a soft spine stabilization system that replicates the physiologic response of a healthy motion segment.